Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 docs/conf.py           |  2 +-
 docs/meson.build       | 21 ++++++++++-------
 docs/sphinx/depfile.py | 51 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 9 deletions(-)
 create mode 100644 docs/sphinx/depfile.py

diff --git a/docs/conf.py b/docs/conf.py
index d6e173ef77..0dbd90dc11 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -67,7 +67,7 @@ needs_sphinx = '1.6'
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['kerneldoc', 'qmp_lexer', 'hxtool']
+extensions = ['kerneldoc', 'qmp_lexer', 'hxtool', 'depfile']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
diff --git a/docs/meson.build b/docs/meson.build
index 20fc92e2fe..08c9139fbe 100644
--- a/docs/meson.build
+++ b/docs/meson.build
@@ -34,16 +34,21 @@ if build_docs
   sphinxmans = []
   foreach manual : manuals
     private_dir = meson.current_build_dir() / (manual + '.p')
+    output_dir = meson.current_build_dir() / manual
     input_dir = meson.current_source_dir() / manual
-    sphinxdocs += custom_target(manual + ' manual',
-                build_always_stale: true,
+
+    this_manual = custom_target(manual + ' manual',
                 build_by_default: build_docs,
-                output: manual,
-                command: [SPHINX_ARGS, '-b', 'html', '-d', private_dir,
-                          input_dir, meson.current_build_dir() / manual])
+                output: [manual + '.stamp', manual],
+                input: [files('conf.py'), files(manual / 'conf.py')],
+                depfile: manual + '.d',
+                command: [SPHINX_ARGS, '-Ddepfile=@DEPFILE@',
+                          '-Ddepfile_stamp=@OUTPUT0@',
+                          '-b', 'html', '-d', private_dir,
+                          input_dir, output_dir])
+    sphinxdocs += this_manual
     if build_docs and manual != 'devel'
-      install_subdir(meson.current_build_dir() / manual,
-                     install_dir: config_host['qemu_docdir'])
+      install_subdir(output_dir, install_dir: config_host['qemu_docdir'])
     endif
 
     these_man_pages = []
@@ -54,9 +59,9 @@ if build_docs
     endforeach
     if these_man_pages.length() > 0
       sphinxmans += custom_target(manual + ' man pages',
-                         build_always_stale: true,
                          build_by_default: build_docs,
                          output: these_man_pages,
+                         input: this_manual,
                          install: build_docs,
                          install_dir: install_dirs,
                          command: [SPHINX_ARGS, '-b', 'man', '-d', private_dir,
diff --git a/docs/sphinx/depfile.py b/docs/sphinx/depfile.py
new file mode 100644
index 0000000000..277fdf0f56
--- /dev/null
+++ b/docs/sphinx/depfile.py
@@ -0,0 +1,51 @@
+# coding=utf-8
+#
+# QEMU depfile generation extension
+#
+# Copyright (c) 2020 Red Hat, Inc.
+#
+# This work is licensed under the terms of the GNU GPLv2 or later.
+# See the COPYING file in the top-level directory.
+
+"""depfile is a Sphinx extension that writes a dependency file for
+   an external build system"""
+
+import os
+import sphinx
+
+__version__ = '1.0'
+
+def get_infiles(env):
+    for x in env.found_docs:
+        yield env.doc2path(x)
+        yield from ((os.path.join(env.srcdir, dep)
+                    for dep in env.dependencies[x]))
+
+def write_depfile(app, env):
+    if not env.config.depfile:
+        return
+
+    # Using a directory as the output file does not work great because
+    # its timestamp does not necessarily change when the contents change.
+    # So create a timestamp file.
+    if env.config.depfile_stamp:
+        with open(env.config.depfile_stamp, 'w') as f:
+            pass
+
+    with open(env.config.depfile, 'w') as f:
+        print((env.config.depfile_stamp or app.outdir) + ": \\", file=f)
+        print(*get_infiles(env), file=f)
+        for x in get_infiles(env):
+            print(x + ":", file=f)
+
+
+def setup(app):
+    app.add_config_value('depfile', None, 'env')
+    app.add_config_value('depfile_stamp', None, 'env')
+    app.connect('env-updated', write_depfile)
+
+    return dict(
+        version = __version__,
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )
-- 
2.26.2